Seawater faraday cage

ABSTRACT

A method for deploying a lightweight, flexible Faraday cage around a device can include the step of directing the conductive fluid flow in a manner that causes a shroud to form over the device. In some embodiments, a flexible material such as canvas can be deployed over the device and the conductive fluid can be sprayed onto the flexible material to form the shroud. In other embodiments, a plurality of nozzles can be placed around the perimeter of the device, and the nozzles can be directed at a predetermined point over the device. The streams can meet at the predetermined point, collide and thereby provide the conductive shroud for the device. The shroud can have a skin depth, which can be chosen according to the desired frequency of electromagnetic radiation to be blocked, typically from one to one hundred millimeters (1-100 mm).

This application is a divisional application of U.S. patent applicationSer. No. 14/038,942, filed Sep. 27, 2013 by Daniel W. Tam et al., for aninvention entitled “Seawater Faraday Cage”. The '942 application isassigned to the same assignee as the present invention.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention (Navy Case No. 103889) is assigned to the United StatesGovernment and is available for licensing for commercial purposes.Licensing and technical inquires may be directed to the Office ofResearch and Technical Applications, Space and Naval Warfare SystemsCenter, Pacific, Code 72120, San Diego, Calif. 92152; voice (619)553-5118; email ssc par T2@navy.mil.

FIELD OF THE INVENTION

The present invention pertains generally to the design of Faraday cages.More specifically, the present invention pertains to methods fordesigning Faraday shields that use conductive fluids to protect vesselcommunications and sensors from the damaging effects caused by radiationand natural lightning.

BACKGROUND OF THE INVENTION

A Faraday cage is a shield formed by conducting material such as metalto protect the enclosure from external electromagnetic radiations. Whenan external electrical field is present, the Faraday cage prevents theelectric field from penetrating within the cage. If the cage isgrounded, the excess charge will flow to ground instead of residing onits outer surface. Traditionally Faraday cages are made of copper,aluminum foil or other metals.

Typically, the effectiveness of the Faraday cage in preventelectromagnetic radiation from passing through is dictated by theconductivity of the cage material and the thickness of the metal.Because Faraday cages are typically made of metal, and/or a metal meshincorporated within a matrix, they are bulky and difficult to movearound. Furthermore, it is not practical to implement a traditionalFaraday cage on a ship to shield the whole ship and its antennas due tosize and weight constraints. Additionally, a Faraday cage will preventthe passage of electromagnetic radiation through the cage in bothdirections, both inbound and outbound. Although there are times when avessel may want to protect itself from inbound electromagneticradiation, there is also a need for a vessel to selectively emit radiowaves, radar emissions, etc. in the conduct of its daily operations.Thus, it is also desirable to have a Faraday cage which can beselectively activated and deactivated.

In view of the above, it is an object of the present invention toprovide a Faraday cage and method for deployment, which uses seawater toprovide the shielding effect. Another object of the present invention isto provide a Faraday cage and method for deployment that is extremelylightweight relative to the amount of area/volume it is designed toprotect. Still another object of the present invention is to provide aFaraday cage and method for deployment that can be selectively activatedand deactivated. Another object of the present invention is to provide aFaraday cage and method for deployment where the size and correspondingarea of coverage can be adjusted during operation of the Faraday cage.Yet another object of the present invention is to provide a Faraday cagewhose protective properties can be adjusted according to the level ofelectromagnetic radiation desired by the user. Another object of thepresent invention is to provide a Faraday cage and method for deploymentthat is easy to use in a cost-efficient manner.

SUMMARY OF THE INVENTION

A method for deploying a lightweight, flexible Faraday cage around adevice can include the initial steps of establishing a flow ofconductive fluid, and directing the conductive fluid flow in a mannerthat causes a shroud of conductive fluid to form over the device. Insome embodiments, a flexible material such as canvas can be spread overan umbrella-like skeletal structure, and the conductive fluid can besprayed onto the flexible material to form the shroud around the device.The shroud of conductive fluid can have a thickness to thereby establishthe attenuation or shielding effect with respect to the device.

The flexible material can also be divided up into portions, which can beplaced over the device, so that a first portion is over the device and alarger second portion of flexible material is over the first portion. Aflow of conductive fluid can then established over both the firstportion and the second portion to form multiple shrouds having first andsecond thickness, so that the shrouds can appear to be concentric whenviewed in plan view. In still other embodiments, a plurality of nozzlescan be placed around the perimeter of the device, and the nozzles can bedirected at a predetermined point over the device. When conductive fluidflow is established through the nozzles, the streams can meet at thepredetermined point and collide to thereby establish the conductiveshroud for the device.

For all of the above embodiments, the shroud(s) can have a thickness,which can be chosen according to the desired frequency ofelectromagnetic radiation to be blocked. Typically, the thickness(s) canbe from one to one hundred millimeters (1-100 mm). The shroud can beformed from any conductive fluid, such as tap water, saltwater, seawateror distilled water, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the present invention will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similarly-referenced characters refer tosimilarly-referenced parts, and in which:

FIG. 1 is a diagram showing the relationship between an incident powerwave, P^(inc), reflected power wave, P^(ref), and transmitted powerwave, P^(trans) at normal angle between two semi-infinite mediums: airand seawater;

FIG. 2 is a graph of the attenuation loss for a propagating wave inseawater;

FIG. 3 is a graph of the transmission loss from a power wave travelingfrom an air medium to seawater;

FIG. 4 is a graph of the total loss for various distances into theseawater;

FIG. 5 is a top plan view of a seawater Faraday cage of the presentinvention, according to several embodiments;

FIG. 6 is a side elevational view of the seawater Faraday cage of FIG.5;

FIG. 7 is a side elevational view of an alternative embodiment of theseawater Faraday cage of FIG. 5;

FIG. 8 is a top plan view of still another alternative embodiment of theseawater Faraday cage of the present invention; and,

FIG. 9 is a block diagram, which illustrates steps that can be taken topractice the methods of the present invention according to severalembodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In brief overview, the seawater Faraday cage of the present inventionaccording to several embodiments can take advantage of the electricalconductivity of the sodium and chloride ions in seawater to create aflexible type of Faraday cage or shield. The conductivity of theseawater can determine the performance of the shielding effectiveness.By using a seawater Faraday cage instead of a traditional metalshielding cage, the weight can be significantly reduced. The seawaterFaraday cage can also be selectively activated and deactivated to avoidinterfering with other operations (outward electromagnetic emissionssuch as radar and radio waves). The fact that seawater can be easilyaccessed from ocean can render a seawater Faraday cage very useful forNaval vessel applications.

In cases where it can be desirable to block electromagnetic radiationfrom impinging on a ship (or any other device or structure), a flow ofconductive fluid can be manipulated to establish a shroud of seawater,which can cover the whole ship or any sections thereof, or any device 10that is mounted on the skin of the ship, and can prevent any damagingeffect due to incoming electromagnetic radiation. The shroud of seawatercan create the Faraday cage. The thickness of the shroud required candepend on the electrical properties of seawater. The electricalproperties of seawater vary in frequency, temperature, and salinity.

FIG. 1 is an illustration showing a power wave traveling in air atnormal incidence that encounter a seawater interface. At incidence, afraction of the power wave will reflect back into the air and the otherpart will be transmitted into the seawater. The power wave transmittedinto the seawater will be attenuated by the properties of seawater asthe power wave propagates through the medium. This power loss is knownas the attenuation loss and the loss due to the reflected power is knownas the transmission loss. The ratio of the power transmitted into theseawater and the incident power in air is given by equation (1):

$\begin{matrix}{\frac{P^{trans}}{P^{inc}} = {\underset{\underset{{Attenuation}\mspace{14mu}{Loss}}{︸}}{{\mathbb{e}}^{({{- 2}\;\alpha\; z})}}{\underset{\underset{{Transmission}\mspace{14mu}{Loss}}{︸}}{{T}^{2}\eta_{1}{{Re}\left( \frac{1}{\eta_{2}^{*}} \right)}}.}}} & (1)\end{matrix}$where α is the attenuation constant in Nepers per meter (Np/m), Re isthe real number component, η₁, is the intrinsic impedance of air (ohms),η₂ is the intrinsic impedance of seawater (ohms), z is the distanceinside the seawater (m), and T is the transmission coefficient. Thetransmission coefficient is calculated by equation (2):

$\begin{matrix}{T = {\frac{2\;\eta_{1}}{\eta_{1} + \eta_{2}}.}} & (2)\end{matrix}$The intrinsic impedance is given by

$\begin{matrix}{{\eta = \sqrt{\frac{{j2}\;\pi\; f\;\mu}{\sigma + {j\; 2\;\pi\; f\; ɛ}}}},} & (3)\end{matrix}$Where j is the imaginary component part, f is the frequency in Hz, μ isthe permeability, ε is the permittivity, and σ is the conductivity. Thepermeability can be expressed as the product of the permeability of freespace and the relative permeability of the material, μ=μ₀×μ_(r), whereμ₀=4π×10⁻⁷ l H/m and μ_(r) is the relative permeability. For air andseawater, μ_(r)=1. The permittivity can be expresses as the product ofthe permittivity of free space and the relative permittivity of thematerial, ε=ε₀×ε_(r), where ε₀=8.854×10⁻¹² f/m and ε_(r) is the relativepermittivity. For air ε_(r)=1 and ε_(r) varies for seawater. Air is adielectric and therefore has a conductivity of zero while theconductivity of seawater varies. The intrinsic impedance of air, η₁, istherefore, 377Ω. The attenuation constant is calculated using equation(4):

$\begin{matrix}{\alpha = {2\;\pi\; f\sqrt{\mu_{2}ɛ_{2}}{\left\{ {\frac{1}{2}\left\lbrack {\sqrt{1 + \left( \frac{\sigma_{2}}{2\;\pi\; f\; ɛ_{2}} \right)} - 1} \right\rbrack} \right\}^{1/2}.}}} & (4)\end{matrix}$The conductivity and permittivity of sea water vary, however, using atypical conductivity value of 4 Siemens per meter (S/m, where a Siemenis the inverse of an Ohm, S=1/Ω) and relative permittivity value of 81,the attenuation loss and transmission loss is calculated to demonstratethe blockage effect for seawater.

FIG. 2 is a plot of the attenuation loss in dB/cm versus frequency. Theplots shows that the attenuation loss increases as the frequencyincreases and the amount of attenuation may be controlled by varying theshroud wall thickness. FIG. 3 is a plot of the transmission loss versusfrequency. As can be seen from FIG. 3, the transmission losses decreaseas the frequency increases. These plots can be used to design the shroudwall thickness required for various frequencies and required shieldingeffect (it should be appreciated that the same analysis could beconducted for salt water, tap water, distilled water or any otherconductive fluid, provided the conductivity and permittivity of theconductive fluid is known). FIG. 4 is a plot showing the total loss(absorption and transmission) for various distances inside the seawater.In addition to this analysis, surface roughness and additionaltransmission loss due to the finite thickness of the shroud needs to beconsidered in enactment.

In cases where multiple shrouds can be envisioned, the spacing betweeneach shroud can also determine the effectiveness of the Faraday shield,in addition to the thickness of respective multiple shrouds. However,the gap between the concentric shrouds is needed to form multiple layersto establish the attenuation effect and achieve a complete Faradayshield.

Referring now to FIGS. 5-7, the seawater Faraday cage 50 of the presentinvention according to several embodiments is shown and is illustrated.As shown, the cage 50 can include a flexible material 52, which can bedraped over a collapsible, umbrella-like framework 53 (shown in phantomin FIG. 5). Collapsible framework 53 can be large enough to cover device10, or even ship 59 in FIG. 5. Or, multiple collapsible frameworks 53can be used to cover the entire ship 59. For clarity, only one framework53 is shown in FIG. 5. A flow of conductive fluid can establish a shroud54 of conductive fluid, which can cover all or part of the ship and/ordevice 10 to be protected.

In some embodiments, and as can be seen from FIGS. 6-7, the flexiblematerial 52 (such as canvas, for example) can be divided into a firstportion 52 a and a second portion 52 b, which can be arranged overrespective collapsible framework 53 a, 53 b so that first portion 52 ais over device 10 and second portion 52 b is over first portion 52 a. Asshown second portion 52 b can have a surface area that can be greaterthan that of first portion 52 a. With this configuration, when flow ofconductive fluid is established, the corresponding shrouds 54 a and 54 bcan be established so that first portion 52 a would be over device,shroud 54 a would enclose the device, second portion 52 b would be overshroud 54 a (and device) and shroud 54 b would enclose shroud 54 a (anddevice 10). Moreover, shrouds 54 a and 54 b would appear to beconcentric when viewed in plan view. To accomplish the seawater flow asdescribed above, a pump 56 can direct conductive fluid through piping 58and through openings (not shown) above frameworks 53 a and 53 b. For theembodiment shown in FIG. 6, there can be a single run of piping 58.Alternatively, piping 58 could branch in a manner that allows for afraction of conductive to flow over first portion 52 a and secondportion 52 b, with sufficient flow rate to establish thickness 60 a and60 b for corresponding shrouds 54 a and 54 b.

Referring now to FIG. 8, several alternative embodiments of the presentinvention can be illustrated. As shown, a plurality of nozzles 62 athrough 62N, can be established around the perimeter of ship 59 and/oror device 10 to be protected. The nozzles 62 can be constructed withsteel, copper or brass in a variety of diameters and heights toaccommodate the requirement, and the nozzles 62 can be oriented todirect a flow of conductive fluid to a predetermined point 64. Point 64can be chosen so that when the streams for nozzles 62 collide, theshroud is established. These embodiments can obviate the need for aframework 53 and flexible material 52 to create shroud 54. Or, thenozzle arrangement could be used in conjunction with framework 53 andflexible material 52 in FIG. 5, instead of piping. Point 62 could bechosen above flexible material 52 and can also be at the geometriccenter of flexible material 52 (when viewed in top plan, as illustratedin FIG. 5) to thereby establish the shroud 54.

With the above configurations, the seawater Faraday cage of the presentinvention can be flexible and light weight, when compared to atraditional metal Faraday cages. The seawater Faraday cage of thepresent invention according to several embodiments can be selectivelyactivated with a flip of switch. In addition, seawater can be used as aninexhaustible supply of conductive fluid without requiring a return lineof fluid, i.e. the seawater Faraday cage of the present invention can bean open system in some embodiments, as seawater can be obtained easilyfrom the ocean, the Faraday cage of the present invention can beestablished, and the seawater can drain overboard during the operationof the seawater Faraday cage, which can be very desirable for Navalapplications. It should also be appreciated that the structure andcooperation of structure described above could also be used to provide afluid Faraday cage over a building or home, providing a conductive fluidsource (most likely public works fireman pressure) is available.

Referring now to FIG. 9, a block diagram 100 is provided to illustratesteps that can be taken to accomplish the methods of the presentinvention. As shown, the methods can include the initial step 102 ofestablishing a flow of conductive fluid 102, using the structure andcooperation of structure described above. The methods can furtherinclude the additional optional step of spreading a flexible materialover the device to be protected, as shown by step 104. The flexiblematerial can be selectively divided into portions and the flow can bedivided as described above to establish concentric shrouds, or it can bespread as a single unitary sheet over collapsible framework, asdescribed above.

The methods of several embodiments can further include the step 106 ofdirecting the conductive fluid to establish a shroud 54 over the device10. Step 106 can be accomplished using the arrangement of piping 58described above, or the aforementioned plurality of nozzles 62 can bedirected at point 64 (with or without flexible material 52 and framework53), also as described above. Step 106 can be accomplished to establisha shroud, or multiple shrouds, that can have thickness, which can befurther selected according to the desired frequency of electromagneticradiation that can be desired to block.

The use of the terms “a” and “an” and “the” and similar references inthe context of describing the invention (especially in the context ofthe following claims) is to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. A Faraday cage for a device, said device having aperimeter, said Faraday cage comprising: a flexible material deployedover said device; a plurality of nozzles around said perimeter; and,said plurality of nozzles directing a conductive fluid flow onto saidflexible material to establish a conductive fluid shroud over saiddevice.
 2. The Faraday cage of claim 1, wherein said conductive fluid isselected from the group consisting of tap water, saltwater, seawater anddistilled water.
 3. The Faraday cage of claim 1, wherein said conductivefluid shroud has a first thickness.
 4. The Faraday cage of claim 3,wherein said first thickness is chosen according to a desired frequencyto be blocked.
 5. The Faraday cage of claim 3, wherein said firstthickness is between 1-100 millimeters (1-100 mm).
 6. The Faraday cageof claim 1, wherein said flexible material further comprises a firstportion and a second portion; said first portion being located betweensaid second portion and said device; said first portion having a firstsurface area and said second portion having a second surface area thatis larger than said first surface area.
 7. The Faraday cage of claim 6,wherein said second portion covers said first portion, and said firstportion covers said device.
 8. The Faraday cage of claim 7, wherein saidfirst thickness is established between said first portion and saidsecond portion, and wherein a fraction of said conductive fluid isdiverted to establish a second conductive fluid shroud having a secondthickness, so that said second portion is between said first thicknessand said second thickness.